The present invention relates to a process for the preparation of hydrogen peroxide by direct synthesis, wherein hydrogen and oxygen are reacted in the presence of a noble metal catalyst, which is bonded to a support or is support-free and is arranged in a fixed bed, and an aqueous-organic or organic solvent, in particular an alcoholic solvent, containing a halide and an acid. The invention also relates to the integration of the process according to the invention into oxidation processes.
It is known to prepare hydrogen peroxide by the anthraquinone process. In this, an anthraquinone derivative is hydrogenated in an organic phase, the intermediate product is oxidized with an oxygen-containing gas in a subsequent stage, and the product is then extracted from the organic phase with water or dilute hydrogen peroxide solution. The aqueous hydrogen peroxide solution obtained in this way is conventionally concentrated to a concentration in the range from 30 to 80 wt. %.
While aqueous hydrogen peroxide solutions are employed in bleaching processes in papermaking and in waste water purification processes, for carrying out oxidation processes on organic substrates using hydrogen peroxide in the presence of a catalyst it is often necessary first to convert the aqueous hydrogen peroxide solution into an organic phase. There is accordingly increasingly a need for organic or organic-aqueous hydrogen peroxide solutions which can be fed directly, that is to say without prior isolation of an aqueous hydrogen peroxide solution, to an oxidation process which is carried out substantially in an organic phase.
The teaching of EP 0 978 316 A1 is an integrated process for a catalytic oxidation process on an organic substrate, the first stage of which comprises preparation of hydrogen peroxide by direct synthesis from hydrogen and oxygen in the presence of a noble metal catalyst, which is bonded to a support, and an organic solvent. The organic hydrogen peroxide solution obtained in this first stage is brought into contact, in an immediately subsequent second stage in the presence of an oxidation catalyst, with the organic substrate to be oxidized. In the context of working up of the reaction mixture of the oxidation stage, the solvent employed in the H2O2 direct synthesis is recovered and recycled into the first stage.
The catalyst for the direct synthesis is a metal of group VIII, in particular palladium, on an active charcoal support which contains sulfonic acid groups. The catalyst is employed in suspended form, namely in an amount in the range from 10−6 to 10−2 mol of metal per 1 of reaction medium.
The solvent in the direct synthesis is preferably methanol. The organic substrates to be oxidized are olefins, aromatic hydrocarbons, ammonia and carbonyl compounds, which are oxidized in the presence of a titanium silicalite catalyst.
A disadvantage of this process is that the support material employed for the catalyst in the direct synthesis is a modified active charcoal, the preparation of which is expensive. The process furthermore requires expensive filtration and recycling measures for the suspension catalyst. In an industrial plant for the direct synthesis of hydrogen peroxide using a catalyst bed of this catalyst bonded to active charcoal instead of the corresponding suspension catalyst, an undesirable and, where appropriate, uncontrolled oxidation of the active charcoal by means of a percarboxylic acid formed as a by-product could occur and could lead to an increase in the potential risks.
A process which is likewise integrated and comprises a direct synthesis for the preparation of hydrogen peroxide and an epoxidation of an olefin is known from DE-OS 198 571 37 A1. The direct synthesis is carried out here in an alcoholic or aqueous-alcoholic medium, in particular methanol, which contains, as a stabilizer, a mineral acid and an alkali halide. According to the examples, methanolic hydrogen peroxide solutions with an H2O2 content in the range from 2 to 6 wt. % can be obtained using a suspension catalyst and an O2/H2 gas mixture (92:8) and can be employed in the epoxidation stage. A disadvantage of this process is that the high H2O2 concentrations mentioned evidently can be obtained only if a gas stream of oxygen and hydrogen, which is already in the explosive range, is passed through the reaction medium. In industrial processes, however, attempts are as a rule made to work outside the explosive range, in order to avoid expensive safety devices for carrying out the process.
In U.S. Pat. No. 5,840,934, which likewise relates to the direct synthesis for the preparation of hydrogen peroxide and a subsequent epoxidation of an olefin, the H2O2 concentration of the methanolic hydrogen peroxide solution obtained in the direct synthesis in the presence of a palladium catalyst and sulfuric acid and sodium bromide as stabilizers is stated as 0.15 and 0.35 wt. % respectively. In this case the direct synthesis takes place outside the explosion limit. Because of the low H2O2 concentration in the methanolic hydrogen peroxide solution, the epoxidation is carried out in the presence of a large amount of the solvent, which leads to a low space/time yield and therefore increased operating costs.
U.S. Pat. No. 4,336,238 describes a process for the preparation of H2O2 solutions in an organic-aqueous solution in the presence of a Pd/C supported catalyst, which is used as a suspension catalyst or as a fixed bed catalyst. The gas mixture of H2 and O2 mentioned in the examples is always in the explosive range, which is a substantial disadvantage. A high productivity (kg H2O2/kg Pd·h) is indeed achieved with a fixed bed catalyst under the operating conditions chosen, but the H2O2 concentration is below 2 wt. %. Further disadvantages of this process are the rapid deactivation of the catalyst and the discharge of noble metal. This document also describes the influence of the acid concentration on the selectivity, the H2O2 concentration and the activity of the catalyst. The activity of a deactivated catalyst can be increased again by significantly increasing the acid concentration of the aqueous-organic medium and/or the reaction temperature over a period of several hours. After this partial activation, the acid concentration and/or temperature can be taken back to a value such as is favourable in respect of the preparation of a solution with a higher H2O2 concentration. Disadvantages of this process are the requirements of having to take expensive measures for reactivation of the catalyst and recovery of noble metal discharged with the H2O2 solution, and the safety problems caused by the active charcoal support material.
According to EP 0 049 806 A1, the H2O2 yield can be increased and the discharge of Pd reduced in the direct synthesis known from U.S. Pat. No. 4,336,238 if methanol is used as the solvent instead of water. However, to stabilize the hydrogen peroxide in methanol, formaldehyde is added to this. The presence of formaldehyde in the methanolic hydrogen peroxide solution, however, is a disadvantage in respect of the use of the organic hydrogen peroxide solution obtained in this manner as an oxidizing agent, because side reactions and/or a reduction in the yield with respect to hydrogen peroxide can occur.
According to DE-OS 196 427 70, organic hydrogen peroxide solutions with a hydrogen peroxide content of at least 2.5 wt. % can be obtained by direct synthesis using H2/O2 mixtures outside the explosive range by carrying out the reaction in the presence of a C1 to C3-alkanol as the reaction medium on specific shaped bodies of catalyst, in particular monolithic shaped bodies. For the preparation of the methanolic hydrogen peroxide solutions with 5.6 to 7 wt. % hydrogen peroxide given by way of example, however, a gas mixture which falls in the explosive range is passed over the catalyst. The monolithic shaped bodies of catalyst are expensive. As can be seen from examples 1 and 2, a severe deactivation of the catalyst occurs in this process during continuous operation.
An object of the present invention is to provide a further process for the direct synthesis of hydrogen peroxide which allows the preparation of aqueous-organic and organic hydrogen peroxide solutions with an H2O2 content of at least 3 wt. %, but for the preparation of which the use or the occurrence of gas mixtures in the explosive range can be reliably avoided.
Another object relates to designing the process such that the catalyst in the reaction of hydrogen and oxygen has a high selectivity and at the same time a high service life and high catalyst productivity.
According to another object, the process should also be suitable for integration into a process for the oxidation of an organic substrate with hydrogen peroxide.